Heat and strain measurements at the crack tip of filled rubber under cyclic loadings using full-field techniques

International audienceThis study aims at characterizing heat sources during the deformation of the crack tip zone in carbon black filled Styrene Butadiene Rubber (SBR). For this purpose, the thermomechanical response of cracked specimens was investigated using coupled full thermal and kinematic field measurements and a suitable motion compensation technique. The kinematic analysis enabled us to define the zone of influence of the crack and to measure the maximum stretch ratio level. The maximum stretch ratio level at the crack tip is higher than that measured at specimen failure during uniaxial tensile tests, which can be explained by considering the maximum chain extensibility. The calorimetric analysis shows that the high heat source gradient zone is very much more confined than the high temperature gradient zone. The heat sources at the crack tip remain positive and small during unloading, which indicates that mechanical dissipation is high and confined to the crack tip. This result highlights that the material behaves very differently in the crack tip zone compared to homogeneous tests. This proves that it not possible to predict the behavior of the crack tip zone from homogeneous tests. Moreover, it is observed that the mechanical dissipation decreases with the number of first cycles, which highlights the fact that the material is increasingly accommodated. This study provides the first accurate measurement of heat sources at the crack tip of rubber, constituting a new experimental tool in the fracture mechanics of rubber

Modelling avalanches in martensites

Solids subject to continuous changes of temperature or mechanical load often exhibit discontinuous avalanche-like responses. For instance, avalanche dynamics have been observed during plastic deformation, fracture, domain switching in ferroic materials or martensitic transformations. The statistical analysis of avalanches reveals a very complex scenario with a distinctive lack of characteristic scales. Much effort has been devoted in the last decades to understand the origin and ubiquity of scale-free behaviour in solids and many other systems. This chapter reviews some efforts to understand the characteristics of avalanches in martensites through mathematical modelling.Comment: Chapter in the book "Avalanches in Functional Materials and Geophysics", edited by E. K. H. Salje, A. Saxena, and A. Planes. The final publication is available at Springer via http://dx.doi.org/10.1007/978-3-319-45612-6_

Stressed microstructures in thermally induced M9R-M18R martensites

We revisit the phase transformation that produces \u2018long-period stacking\u2019 M9R\u2013M18R martensites in Cu-based shape-memory alloys and analyze some associated microstructures, in particular, the typical wedge-shaped configuration. Our basic premise is that the cubic-to-monoclinic martensitic phase change in these alloys is, geometrically, but a slight modification of the well-known bcc-to-9R transformation occurring in various elemental crystals, whose lattice strain is, at the microlevel, the same Bain strain as for the bcc-to-fcc transformation. For the memory alloys we thus determine the \u2018near-Bain\u2019 microstrain, thereby analyzing the faulted, long-period stacking martensite as a mesoscale structure derived from compatibility with the austenite. We compute the transformation-twin systems, habit planes, average deformation and stacking-fault density of the 9R, 18R, M9R or M18R martensites, as they arise from the compatibility conditions between the parent and product lattices. We confirm earlier conclusions that a stress-free wedge is not kinematically compatible in these materials. However, we show that this microstructure is \u2018close enough\u2019 to compatibility, finding that its stress levels are low and should cause only minimal plastification and damage in the crystal. The wedge is therefore rationalized as a viable path for the transformation also in these substances. We verify this to hold for all the lattice parameters reported for Cu-based alloys. In general, we conclude that martensitic microstructures can be stressed to a degree also in good memory materials. Furthermore, we find that the lattice-parameter relations, guaranteeing the zero-stress compatibility of special configurations favoring the transformation and its reversibility, do not need to be strictly enforced in these crystals, because the residual stresses in microstructures are low regardless of lattice-parameter values

Conductivité thermique et capacité thermique spécifique de l'austénite et de la martensite induite par la contrainte dans le NiTi superélastique à température ambiante

International audienceInfrared thermography (IRT) and heat source reconstruction (HSR) were used in the study to measure two thermophysical properties of superelastic nickel-titanium (NiTi) shape memory alloy (SMA) wires, namely thermal conductivity and specific heat capacity. Since the values are potentially phase dependent, the identification was carried out at ambient temperature with and without mechanical loading, i.e., in the stress-induced martensite state and in the austenite state, respectively. A uniaxial testing machine was used to apply constant deformation while allowing temperature measurements by IRT during Joule heating and natural return the thermal equilibrium. The data processing by HSR, including preliminary filtering operations, was preliminarily evaluated from temperatures provided by a model (with added noise). It gave an error of ± 0.5% for the thermal conductivity and ± 1% for the specific heat capacity. The experimental analysis showed that the thermal conductivity of stress-induced martensite is 26% higher than that of austenite at the same temperature (here the ambient temperature). Regarding the specific heat capacity, that of stress-induced martensite is 4.7% lower than that of austenite. Explanations for these differences were proposed from solid-state physics theory. The measured values were also compared to data collected in the literature (obtained at zero stress at different temperatures).La thermographie infrarouge (IRT) et la reconstruction de la source de chaleur (HSR) ont été utilisées dans cette étude pour mesurer deux propriétés thermophysiques des fils d'alliage à mémoire de forme (SMA) superélastiques nickel-titane (NiTi), à savoir la conductivité thermique et la capacité calorifique spécifique. Les valeurs étant potentiellement dépendantes de la phase, l'identification a été effectuée à température ambiante avec et sans charge mécanique, c'est-à-dire respectivement à l'état de martensite induite par la contrainte et à l'état d'austénite. Une machine d'essai uniaxiale a été utilisée pour appliquer une déformation constante tout en permettant des mesures de température par IRT pendant le chauffage par effet Joule et le retour naturel à l'équilibre thermique. Le traitement des données par HSR, y compris les opérations de filtrage préliminaires, a fait l'objet d'une évaluation préliminaire à partir des températures fournies par un modèle (avec un bruit ajouté). Il a donné une erreur de ± 0,5 % pour la conductivité thermique et de ± 1 % pour la capacité thermique spécifique. L'analyse expérimentale a montré que la conductivité thermique de la martensite sous contrainte est 26% plus élevée que celle de l'austénite à la même température (ici la température ambiante). En ce qui concerne la capacité thermique spécifique, celle de la martensite induite sous contrainte est inférieure de 4,7 % à celle de l'austénite. Des explications de ces différences ont été proposées à partir de la théorie de la physique de l'état solide. Les valeurs mesurées ont également été comparées aux données recueillies dans la littérature (obtenues à contrainte nulle à différentes températures)

Almost compatible microstructures in shape memory alloys

Coherent stress-free (CSF) microstructures with specific morphologies are favored in shape memory alloys (SMAs) when special relations are satisfied by the lattice parameters. Experimentally observed microstructures are, however, also formed at non-exact CSF conditions. Here we propose a framework for the investigation of almost compatible (i.e. non-perfectly CSF) twinned wedges in SMAs, and make a systematic study of these microstructures for two types of symmetry-breaking martensitic transformations. We determine the domains in lattice-parameter space wherein there exist, and coexist, different families of almost compatible wedges with low overall stress. We find these to be wide regions largely unrelated to the existence of special CSF relations, if any even exist, giving stress-free configurations. We propose SMA improvement can be obtained by targeting domains in lattice-parameter space wherein, besides satisfying other suitable properties, a maximum number of almost compatible microstructures can also form in the material. We develop this approach for wedges in SMAs undergoing the cubic-to-orthorhombic transformation

Optimisation de patchs pour ailes endommagées

International audienceThis paper describes the optimization process of composite patches designed to reinforce damaged wings. The main objective is to reduce the stress level in a damaged area located near the anchorage of the wing to the fuselage. Genetic algorithms are used for optimizing the ply orientations of the stacking sequence as well as the location and shape of the patch. The stress field in the structure is computed with the ANSYS finite element package. Several examples illustrate the fact that the stress flow within the wing is deviated, thus leading to a decrease of the stress level in the damaged area to be relieved

Thermoelastic analysis of a copper-based monocristalline shape-memory alloy

The objective of this study is to observe the thermoelastic coupling effect in a monocrystalline Cu-Al-Be shape-memory alloy. The orientation of the austenite crystal is measured by Microfocus X-ray texture analysis. Uniaxial cyclic loading is applied to the specimen at room temperature. Infrared thermography is used to capture temperature fields on the specimen surface. The approach consists in analysing the cyclic temperature oscillation of points located on the surface of the specimen. The study highlights first the significant difference between the thermoelastic couplings in austenite and martensite. Second the anisotropy of both phases, especially the martensite, is evidenced

Thermoelastic analysis of a copper-based monocristalline shape-memory alloy

The objective of this study is to observe the thermoelastic coupling effect in a monocrystalline Cu-Al-Be shape-memory alloy. The orientation of the austenite crystal is measured by Microfocus X-ray texture analysis. Uniaxial cyclic loading is applied to the specimen at room temperature. Infrared thermography is used to capture temperature fields on the specimen surface. The approach consists in analysing the cyclic temperature oscillation of points located on the surface of the specimen. The study highlights first the significant difference between the thermoelastic couplings in austenite and martensite. Second the anisotropy of both phases, especially the martensite, is evidenced